The present invention relates to an apparatus and a method for controlling fuel injection of an internal combustion engine, and to an internal combustion engine.
In an internal combustion engine such as an automobile engine, a catalytic converter having three-way catalysts is provided in an exhaust passage to purify exhaust gas. Specifically, the three-way catalysts oxidize CO and HC in exhaust gas and reduce NOx, thereby changing these into harmless CO2, H2O, N2. Such purification of exhaust gas using three-way catalysts, that is, oxidation of CO, HC and reduction of NOx, are performed most effectively in a catalyst atmosphere of which the concentration of oxygen corresponds to that of combustion of air-fuel mixture at the stoichiometric air-fuel ratio.
Therefore, in the above described internal combustion engine, air-fuel ratio feedback control is performed in which the actual air-fuel ratio is set to the stoichiometric air-fuel ratio. In the air-fuel ratio feedback control, a feedback correction value that is used for correcting fuel injection amount is changed based on the actual air-fuel ratio such that the actual air-fuel ratio becomes equal to the stoichiometric air-fuel ratio.
That is, when the actual air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the feedback correction value is increased as the actual air-fuel ratio becomes leaner. This increases the fuel injection amount so that the actual air-fuel ratio approaches the stoichiometric air-fuel ratio. Also, when the actual air-fuel ratio is richer than the stoichiometric air-fuel ratio, the feedback correction value is decreased as the actual air-fuel ratio becomes richer. This decreases the fuel injection amount so that the actual air-fuel ratio approaches the stoichiometric air-fuel ratio.
The fuel injection amount of an internal combustion engine is adjusted by changing the valve opening time (actuation time) of the fuel injection valve. The less the fuel injection amount, the shorter the actuation time of the fuel injection valve becomes. However, if the actuation time of the fuel injection valve is excessively short, changes in the fuel injection amount per unit time cannot be maintained constant in relation to changes in the valve opening time of the fuel injection valve per unit time due to the structural problems of the valve. The fuel injection thus becomes unstable.
Accordingly, Japanese Laid-Open Patent Publication No. 60-22053 discloses a technique in which, as a feedback correction value decreases and the actuation time of the fuel injection valve becomes less than a permissible value that permits the fuel injection valve to stably inject fuel, the feedback correction value is fixed to a reference value (initial value), so that the air-fuel ratio feedback control is stopped, and the actuation time of the fuel injection valve is set to the shortest permissible time. In this case, since the actuation time of the fuel injection valve does not stay less than the minimum permissible time, the accuracy of adjustment of the fuel injection amount is prevented from being degraded by unstable fuel injection from the fuel injection valve.
However, when the feedback correction value stays significantly less than the reference value, if the actuation time of the fuel injection valve is temporarily shorter than the permissible minimum time, and then reaches or surpasses the permissible minimum time immediately thereafter, the actual air-fuel ratio becomes rich. This inevitably degrades the emission and the combustion stability. The reason why the actual air-fuel ratio becomes rich under these circumstances will now be explained.
When the actuation time of the fuel injection valve is less than the permissible minimum time, the feedback correction value, which has been staying below the reference value, is fixed to the reference value. In other words, the correction value is increased significantly. At this time, since the actuation time of the fuel injection valve is set to the permissible minimum time regardless of the magnitude of the feedback correction value, the actual air-fuel ratio is not richened due to an excessive fuel injection amount when the feedback correction value is significantly increased as described above.
However, when the actuation time of the fuel injection valve reaches or surpasses the permissible minimum time immediately after the feedback correction value is fixed, the fixation of the actuation time of the fuel injection valve to the permissible minimum time is cancelled, and the actuation time is set to time that corresponds to the fuel injection amount that is adjusted using the feedback correction value. Since the fixation of the feedback correction value to the reference value has just been cancelled and the feedback correction value has just started being changed based on the air-fuel ratio, the feedback correction value is significantly greater than the value immediately before the fixation. Therefore, correction of the fuel injection amount based on the feedback correction value causes the actual air-fuel ratio to be richer than the stoichiometric air-fuel ratio.
Further, after the fixation is cancelled, the feedback correction value starts decreasing toward the value immediately before the fixation through changes based on the actual air-fuel ratio, such that the actual air-fuel ratio becomes equal to the stoichiometric air-fuel ratio. However, since the decrease of the feedback correction value starts from the reference value, the decrease of the correction value takes a long time until the actual air-fuel ratio becomes the stoichiometric air-fuel ratio. Until the time elapses, the actual air-fuel ratio inevitably stays richer than the stoichiometric air-fuel ratio.